With development of computer technologies, a large-capacity memory has been required. To enlarge a capacity of a memory, the number of memory cells may be increased. However, when the number of memory cells is increased, the area is increased, and downsizing of a memory and a computer having the memory is limited.
Such a memory includes a RAM (Random Access Memory) or a ROM (Read Only Memory). The RAM is a rewritable memory, and the ROM is a memory which is readable only.
The RAM and the ROM each have a word line and a data line, and a memory cell is formed at the intersection of these lines. Predetermined information can be made by using binary states (0 and 1) of the memory cell.
As for the ROM, in order to reduce the number of revised masks and improve degree of integration, a structure of a multivalued mask ROM in which potential of the word line is easily controlled at the time of reading is proposed (see Reference 1: Japanese Published Patent Application No. 2000-299394).
In addition, in order to reduce current consumption at the time of selecting the word line, a structure of a ROM in which a data holding unit and a power supply controller are included and the data holding unit has a plurality of memory cells is proposed (see Reference 2: Japanese Published Patent Application No. 2005-203079).